1. Field of the Invention
The present invention relates to new heat curable coating compositions containing butanone oxime-blocked isocyanate prepolymers and cycloaliphatic polyamines and to their use for producing decorative coatings that are resistant to flying stones on a metal substrate, e.g., in the door sill area of a motor car.
2. Description of the Prior Art
Heat curable, one-component primers, which are resistant to flying stones and prepared from blocked NCO prepolymers and diamines, are known. This type of coating composition, described for example in DE-PS 2,131,299, contains aromatic diisocyanates and polyether polyols. In view of these structural components it is obvious that these coatings can really only be used beneath a top coat. If they were used as a top coat, they would turn yellow and chalky in the light.
The same is true of the coating compositions according to EP-A-0,536,614, which are only used as undercoating lacquers or as underbody sealants. It is a common feature of both systems that they use .epsilon.-caprolactam as a blocking agent, which requires stoving temperatures of about 160.degree. C. In practice there is an increasing demand for coatings which are based on blocked polyisocyanates which can be cured at about 140.degree. C.
Coating compositions are known from EP-A-0,059,962, which are based on ketoxime-blocked NCO prepolymers prepared from aliphatic diisocyanates. In the working examples, only NCO prepolymers prepared from polyether polyols are described. The presence of polyether polyols and the disclosed use of the systems for producing undercoats, which are resistant to flying stones, leads to the conclusion that the coating compositions are not intended for use as decorative light-resistant topcoats.
Although in the general description of the publication, polyesterpolyols are also disclosed as suitable polyhydroxyl compounds for producing the NCO prepolymers, they were not considered to be preferred polyols as can be seen from their absence in the working examples.
An object of the present invention is to provide heat curable coating compositions containing blocked NCO prepolymers and cycloaliphatic diamines which combine the following properties:
light resistance for use to produce light-resistant topcoats; PA1 stoving temperatures of ca. 140.degree. C.; PA1 storage stability at 25.degree. C. for a period of at least 6 months. In this case, "storage stability" is understood to be the ability to be applied without any problems, i.e., with at most a negligible increase in viscosity; PA1 suitability for the production of decorative outer coatings and PA1 hard and tough consistency, thereby ensuring suitability for protection against flying stones. PA1 A) prepolymers which have butanone oxime-blocked isocyanate groups and an average molecular weight, calculated from the NCO content and NCO functionality, of 1000 to 10,000, and are prepared from PA1 B) cycloaliphatic polyamines having at least two primary amino groups,
This object may be achieved with the binders according to the invention which are described below and based on NCO prepolymers prepared from selected starting materials.